1. Field of the Invention
The present invention relates to communications systems, and more particularly to communications systems including apparatus and method for communicating with mobile terminals as they move from place to place.
2. Prior Art
A communications network generally consists of switching or routing equipment connected together by either wired or wireless links. Network subscribers or user terminals are connected to at least one switch or router in the network by a communications link which, in the case of a wireless terminal, is a wireless link. The communications network transports signals generated by the user terminal to another user terminal connected to the same communications network. In a packet switching network, the user generated signals are presented and contained in packets. The communications between two or more user terminals requires a route to be established between the involved terminals. At connection set-up time, a set of routers or switching points of the communications network is selected. Each of these routers is configured such that when it receives a packet either from another router or from the originating station, the packet is forwarded to an appropriate link connected to another router or to the destination terminal. All such routers which transport packets generated by a user terminal to a destination station and the communications links which connect them constitute a path or a route for that specific set of communicating stations. There exists many routing algorithms which can be used for the route selection and the way a router makes a decision on choosing an appropriate link to forward packets. For many routing protocols such as internet protocol (IP) and network/IPX, routing mechanisms are based on a number of elements such as periodic topology updates which are received by every router. For each packet received, a router chooses an appropriate link to which the packet is forwarded based on a network routing technique which uses the existing topology information as well as the routing information contained in the packet, the network topology is represented in terms of nodes and the links between them. Each node or link is represented by an address. There is a unique relationship between each node of the network and its associated address. By making all routers of a communications network aware of an entity of the communications network by assigning that entity an address, and by including that address in the topology update function, packets destined to that entity will flow through the communications network and each router then forwards the packets to another router which eventually will result in the delivery of such packets to the addressable entity. In general, these addressable entities known to the routers of the network represent a plurality of stations. A network address is assigned to a number of neighboring stations and each station is identified by a second unique address, for example an IP address or a MAC (Media Access Control) address. In one layer of the routing, the packets destined to the station are routed such that they arrive in the domain of all stations all sharing the same network address. Then, based on the specific MAC or intra domain address, the packets are delivered to the final destination.
Without changing the network routing methodology or the network operating system of the user station, the above described methodology fails in a mobile/wireless network. In a mobile/wireless network, the access point and the addressable network domain of a mobile terminal changes as a result of the handoff of a mobile terminal from one access point to another.
One attempt to solve the problem described above is shown in U.S. Pat. No. 5,940,139. The patent describes a method of routing packets through a fixed source routing communications network to and from mobile units. An initial access point is determined when a communication session is established to or from a mobile unit through the network. Location information is updated each time the mobile unit moves out of the range of one access point and into the range of another access point of the network. When packets for the session are to be transmitted to the mobile unit from the wired network, the packets are forwarded from the initial access point to a current access point which by definition is in the range of the mobile units. When packets for the session are to be transmitted from the mobile unit to the wired network, the access point intercepts these packets and routes them to their destination and/or initiates root discovery to those destinations. When packets are to be transmitted between mobile units at different access points, the packets are sent between the current access points and not through home access points.
Although the above identified patent provides a mechanism for routing packets through a fixed source routing communications network to or from a mobile unit, it requires significant overhead in updating location information for each mobile unit as it moves from one access point in the network into the range of another access point of the network.